A gis represents reality, it is not reality a gis represents reality, it is not reality

 Sana 10.06.2019 Hajmi 445 b. • To determine location of features in real world or on map need a reference system

• A set of lines of known location that can be used to determine the locations of features that fall between the lines • Coordinate Systems

• Reference systems used to determine feature locations
• In this module learn about

• different coordinate systems
• how they work
• how to change the coordinate system of a map.
• Understanding coordinate systems  manage your data to increase accuracy • Change the map projection for a data frame and describe its effects. • Two types of coordinate systems

• Geographic
• Used to locate objects on the curved surface of the earth
• Projected
• Used to locate objects on a flat surface
• a paper map or a digital GIS map displayed on a flat computer screen.
• Each attempts to model earth and feature locations accurately

• But no system is completely accurate • Consists of a network of intersecting lines called a graticule

• Intersecting lines = longitude and latitude • Graticule

• Longitude
• Vertical lines
• Latitude
• Horizontal lines
• Because earth is spherical, these lines form circles • Measurements expressed in

• Degrees
• 1/360th of a circle.
• Can be divided into 60 minutes
• Minutes
• Can be divided into 60 seconds
• Seconds • Lines of longitude

• Called meridians
• Measures of longitude begin at the prime meridian
• Defines zero value for longitude
• Range from 0° to 180° going east
• Range from 0° to -180° going west • Prime meridian

• Green line
• Starting point for longitude
• Has a value of 0
• Equator

• Red line
• Starting point for latitude
• Has a value of 0
• Runs midway between the north and south poles
• Dividing earth into northern and southern hemispheres. • For example, consider these coordinates:

• Longitude: 60 degrees East (60° 00' 00")
• Latitude: 55 degrees, 30 minutes North (55° 30' 00")
• Longitude coordinate refers to angle formed by two lines

• one at the prime meridian
• the other extending east along the equator.
• Latitude coordinate refers to angle formed by two lines

• one on the equator
• the other extending north along the 60° meridian. • Longitude and latitude are angles measured from the earth's center to a point on the earth's surface. • Many models of the earth's shape

• Each has its own geographic coordinate system
• All based on

• degrees of latitude and longitude
• Exact latitude-longitude values assigned to individual locations will vary • Two shapes commonly used to model earth

• Sphere
• Spheroid • Assuming the earth is a sphere greatly simplifies mathematical calculations

• Works well for small-scale maps
• Maps that show large area of the earth
• A sphere does not provide enough accuracy for large-scale maps

• maps that show smaller area of earth in more detail

• A spheroid is a more accurate model of the earth, but it's not perfect. • Planet Earth

• slightly pear-shaped and bumpy
• has several dents and undulations
• south pole is closer to the equator than north pole
• Geoid

• Model for complicated of earth
• Too mathematically complicated to use for practical purposes, so spheroid is used as a compromise • Some spheroids were developed to

• Model the entire earth
• Model specific regions more accurately
• World Geodetic System of 1972 (WGS72) and 1984 (WGS84)

• Used to represent the whole world
• Clarke 1866 and Geodetic Reference System of 1980 (GRS80)

• Most commonly used in North America • Why do you need to know about spheroids?

• Because ignoring deviations and using the same spheroid for all locations on the earth could lead to measurement errors of several meters or, in extreme cases, hundreds of meters. • For this purpose, a geographic coordinate system uses a datum.

• A datum specifies which spheroid you are using as your earth model and at which exact location (a single point) you are aligning that spheroid to the earth's surface. • Red spheroid

• Aligned to the earth to preserve accurate measurements for North America
• Blue spheroid

• Aligned to the earth to preserve accurate measurements for Europe • Datum

• Defines origin of geographic coordinate system
• The point where the spheroid matches up perfectly with the surface of the earth and where the latitude-longitude coordinates on the spheroid are true and accurate.
• All other points in the system are referenced to the origin.
• In this way, a datum determines how your geographic coordinate system assigns latitude-longitude values to feature locations.
• There are different datums to help align the spheroid to the surface of the earth in different regions • For example, consider a location in Redlands, California, that is based on the North American Datum of 1983

• The coordinate values of this location are:
• –117° 12' 57.75961" (longitude) 34° 01' 43.77884" (latitude)
• Now consider the same point on the North American Datum of 1927

• –117° 12' 54.61539" (longitude) 34° 01' 43.72995" (latitude)
• The longitude value differs by about three seconds, while the latitude value differs by about 0.05 seconds. • In both NAD 1927 and the NAD 1983 datums

• Spheroid matches the earth closely in North America
• Is quite a bit off in other areas
• Notice that the datums use different spheroids and different origins

• origin aligns the Clark 1866 spheroid with a point in North America

• Origin aligns the center of the spheroid with the center of the earth • The most recently developed and widely used datum for locational measurement worldwide is

• World Geodetic System of 1984 (WGS 1984) • To convert feature locations from the spherical earth to a flat map

• Latitude and longitude coordinates from a geographic coordinate system must be converted, or projected, to planar coordinates • A map projection uses mathematical formulas to convert geographic coordinates on the spherical globe to planar coordinates on a flat map. • Projected coordinate system

• A reference system for identifying locations and measuring features on a flat (map) surface
• Consists of lines that intersect at right angles, forming a grid
• Based on Cartesian coordinates
• Have an origin, an x and a y axis, and a unit for measuring distance • Feature locations are measured using x and y coordinate values from the point of origin. • The origin of the projected coordinate system

• (0,0)
• commonly coincides with the center of the map.
• This means that x and y coordinate values will be positive only in one quadrant of the map (the upper right).

• On published maps, however, it is desirable to have all the coordinate values be positive numbers. • To offset this problem

• Mapmakers add 2 numbers to each x and y value
• Numbers are big enough to ensure that all coordinate values, at least in the area of interest, are positive values.
• False easting
• Number added to the x coordinate
• False northing
• Number added to the y coordinate • A false northing value of 2,000,000 was added to each y coordinate. • When you add a dataset to ArcMap it detects the geographic coordinate system and the projected coordinate system if there is one • ArcMap can convert the geographic coordinate system to any projected coordinate system and it can convert any projected coordinate system back to the geographic coordinate system. • The layers will overlay properly. • ArcMap can still project the data on the fly, but it can no longer guarantee perfect alignment.

• For perfect data alignment, you need to apply a transformation to make the geographic coordinate systems match • How do you know what coordinate system your data is stored in?

• You can view the coordinate system information for a dataset in ArcCatalog™, in its metadata.
• If a dataset has no coordinate system information in its metadata (it's missing), you may not be able to display the data in ArcMap.
• You may need to do some research to find out the coordinate system, then define the coordinate system using the ArcGIS tools provided. • Map units

• Units in which coordinates for a dataset are stored
• Determined by the coordinate system
• If data is stored in a geographic coordinate system
• Map units are usually decimal degrees
• If data is stored in a projected coordinate system
• Map units are usually meters or feet
• Units can be changed only by changing the data's coordinate system.
• Display unit

• Independent of map units
• Are a property of a data frame
• The units in which ArcMap displays coordinate values and reports measurements.
• You can set the display units for any data frame and change them at any time. • Degrees can be expressed two ways:

• degrees, minutes, seconds (DMS)
• decimal degrees (DD).
• In a GIS, decimal degrees are more efficient because they make digital storage of coordinates easier and computations faster. • View and modify coordinate system information • Map projection

• Used to convert data from a geographic coordinate system to a projected (planar) coordinate system
• There are many different map projections

• Each preserves the spatial properties of data (shape, area, distance, and direction) differently • Maps are always flat, so do you always need a map projection?

• Maybe—it depends on what you want to do.
• For example, suppose your project doesn't require a high level of locational accuracy—you won't be performing analysis based on location and distance or you just want to make a quick map. In these situations, there is probably no need to convert your data to a projected coordinate system. • Use a map projection to convert data to a projected coordinate system

• If you need to perform analysis
• measure distances, calculate areas and perimeters, determine the shortest route between two points
• If you need to show a particular spatial property for features on a map as it really exists on the earth • The term "map projection" comes from the concept of projecting a light source through the earth's surface onto a two-dimensional surface (a map). • Cylinder
• Cone
• Plane
• Each of these surfaces can be laid flat without distortion. • Cylinder

• wrapped around the earth so that it touches the equator
• accurate in the equatorial zone

• Plane

• touches the earth at a pole
• accurate in the polar region.
• Knowing the surface used helps determine if the map projection is right for purpose • Produce maps with

• straight, evenly-spaced meridians
• straight parallels that intersect meridians at right angles
• Created by

• wrapping a cylinder around a globe
• projecting a light source through the globe onto the cylinder
• cutting along a line of longitude
• Being laid flat • Produce maps with

• straight converging longitude lines
• concentric circular arcs for latitude lines
• Created by

• setting a cone over a globe
• projecting light from the center of the globe onto the cone
• cutting along a longitude line • Produce maps on which

• longitude lines converge at the north pole and radiate outward
• Latitude lines appear as a series of concentric circles
• Created by

• passing a light source through the earth onto a flat surface (plane).
• In this example, the plane touches the earth at the north pole. • Shape
• Area
• Distance
• Direction
• Each map projection is good at preserving one or more (but not all) • Different map projections preserve different spatial properties and produce different-looking maps. • Shape

• Shapes, such as outlines of countries, look the same on the map as they do on the earth.
• Called "conformal”
• Compass directions are true for a limited distance around any given location • Area

• Size of a feature on the map is the same relative to its size on the earth
• If you draw a shape and move it around the map, no matter where you place it, its size will be the same • Distance

• A line between one point on the map and another is the same distance as it is on the earth (taking scale into consideration).
• Most maps have one or two lines of true scale.
• An equidistant map preserves true scale for all straight lines passing through a single specified location
• i.e. if the map is centered on Moscow, a linear measurement from Moscow to any other point on the map would be correct • Direction

• Direction, or azimuth, is measured in degrees of angle from north
• preserves direction for all straight lines passing through a single, specified location
• Directions from one central location to all other points on the map will be shown correctly • The azimuth from A to B is 22 degrees. If the azimuth value from A to B is the same on a map as it is on the earth, then the map preserves direction from A to B. • When choosing a map projection, think about which properties you want to preserve.

• If your map is large-scale (shows a relatively small area of the earth), the effect of a map projection will be much less than if your map is small-scale (shows a large portion of the earth's surface). • Which map projection you choose for a particular map depends on

• Map's purpose
• Spatial properties you want to preserve • If map will be used for general reference or in an atlas

• Want to balance shape and area distortion.
• In this case, a compromise projection such as the Robinson projection may be the best choice.
• If map has a specific purpose

• May need to use a projection that preserves a specific spatial property  • Other factors to consider when choosing a map projection

• the size of the area you're mapping,
• the orientation (east-west or north-south)
• the particular portion of the earth that is covered.
• When working at a large scale

• distortion doesn't play a big role
• almost any projection centered on your area will be appropriate
• In some situations, decision of which map projection to use has already been made

• State Plane and UTM are standard for mapping U.S. states • Compare different map projections • As long as they have a common geographic coordinate system, ArcMap can properly display multiple datasets in the same data frame using a process called "on-the-fly projection." • Anyone who uses maps should know which projections are being used and which properties are distorted and to what extent. • What are the two most important factors to consider when choosing a map projection? • When choosing a map projection, you should consider the purpose of the map and which spatial properties you want to preserve. Do'stlaringiz bilan baham:

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